The present invention relates to an electrical device having a substrate of sintered silicon carbide on which a metal film is formed for use in electronic equipment such as substrates for wiring and wave absorbers.
Sintered compacts of silicon carbide are known to be superior for use under severe conditions as they have excellent properties including corrosion-resistance, high thermal conductivity, low thermal expansivity and resistance to thermal shock. However, because the impact resistance of such a sintered compact is lower than that of metal, it is often reinforced with a metal layer which may be formed of a plurality of metal films. For example, one or more than one metal film may be formed into a metal layer on the surface of a ceramic substrate through the high melting point metal technique (known as the Telefunken Method), activated metal method, physical evaporation or plating. Japanese Patent No. 59-57976 discloses "Ceramics Having Laminated Metal Film" invented by the present inventors. In addition, published Japanese Patent Application No. 59-57973 discloses a "Metal-Ceramic Composite Material." According to these references, metals selected from the groups IV B, VI B and IB of elements are applied on the surface of ceramic materials in a predetermined order through physical evaporation or plating to form ceramics having a laminated metal film thereon.
Conventional metal films formed on the surfaces of sintered compacts of silicon carbide by means of the high melting point metal method contain only a small amount of oxide for forming a reaction phase within the silicon carbide. A metal film formed by means of activated metal method allows carbon to be liberated through a eutectic reaction of the activated metal with the silicon in the silicon carbide. A metal film formed by means of the physical evaporation method allows the carbon to be liberated with the thermal expansive reaction. Consequently, when the metal film is formed by plating simultaneously with at least two of the aforementioned methods, the adhesion of the metal is not sufficient for the reasons described in reference to the aforementioned three methods.
The present invention is intended to solve the adhesion problem. It is, therefore, an object of the invention to provide a sintered substrate of silicon carbide having a metal film strongly bonded to the substrate. It is an additional object of the invention to provide a method of forming a device having a sintered silicon carbide substrate with a film of metal strongly bonded to the substrate.